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In this link https://www.dyxum.com/dforum/ayone-knows-the-mc-mto11ca-10-1000-telephoto-lens_topic12136.html A poster claims that the 1000mm mto-11 can be extended to 2000mm by removing a stop and adding a 100mm extension tube.

Is this true for any variant of this lens or is the poster confusing an extension tube with a teleconverter?

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  • \$\begingroup\$ A teleconverter is not expressed in "mm". But the effect of extension tube is really a ratio of their "thickness" to the focal length, and 100mm isn't that much on a 1000mm lens. It appears that this specific lens can focus well beyond infinity so you can focus at infinity even with an extension tube, but I don"t think you get a 2x zoom effect with this. You would just get a 1100mm lens that can lets you get closer to your subject in macrophotography. \$\endgroup\$
    – xenoid
    Aug 9, 2019 at 8:47
  • \$\begingroup\$ @xenoid, I doubt it focuses that far past infinity. I tried adapting my 1100mm mirror lens to a telescope with a short inverting tube, adding about 50mm to the optical path, and couldn't focus farther than a few metres. \$\endgroup\$
    – Peter Taylor
    Aug 10, 2019 at 10:49

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It's at least plausible: a length extension will cause a crop to end up on the sensor/film and thus will cause a loss of light. How much effect an extension has depends on effective diameter and position of the last optical element. Also an extension tube will require a different focus setting. Again, the amount of difference will depend on effective diameter and position of the last optical element. If you are unlucky, the focus range after extension is not generally useful. It will usually work for the purpose of closeup photography. However, how good the results are depends on the optical recipe of the lens since things like flat field correction and chromatical aberration compensation are done with a fixed sensor plane in mind.

For mirror lenses, the strongest optical element may be the mirror which does not need chromatical aberration compensation. At least that may be an advantage.

So the claim is plausible. Whether it is true would require someone with personal experience to corroborate.

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